Acrylic resins are widely used in various fields as molding materials due to their excellent transparency, luster, surface hardness, weatherability, mechanical strength, heat distortion resistance, etc.
Conventional acrylic resins, however, are often subject to the defect of crazing or crack formation when they are brought into contact with an organic solvent, such as ethanol, thinner for paints, etc. Examples of instances when organic solvents may come into contact with acrylic resins with resultant crazing or crack formation include when an antistatic agent is coated on an acrylic resin molding using ethanol as a diluent, or when the resins are subjected to printing or coating using an organic solvent as a thinner, or when acrylic resin plates are bend-fabricated and adhered to each other using methylene chloride or the like. Since crazing or crack formation spoils the excellent appearance of acrylic resins, and since their excellent appearance is one of their substantial merits, crazing or crack generation in acrylic resins is a substantial problem.
The resistance of acrylic resins to occurrence of crazing or crack formation when they are brought into contact with an organic solvent (referred to herein as "solvent resistance") depends upon the kinds and proportions of copolymerized monomers, average molecular weight, and the kinds and amounts of various additives, such as lubricants or plasticizers, added to the resins for various purposes. In general, the solvent resistance increases as the molecular weight of the resin increases. However, if the molecular weight becomes too high, melt viscosity becomes so high that workability decreases, making molding difficult. Hence molecular weight can not be increased without restriction.
In order to reduce the melt viscosity for molding workability, methyl methacrylate (hereinafter abbreviated as MMA) has been copolymerized with acrylic ester (hereinafter abbreviated as AE) and/or a methacrylic ester (hereinafter abbreviated as MAE) consisting of the reaction product of an alkyl alcohol having two or more carbon atoms and methacrylic acid. It is also known to reduce melt viscosity by adding a lubricant or a plasticizer, such as stearyl alcohol, cetyl alcohol, dibutyl phthalate, 2-ethylhexyl phthalate, etc. However, these methods decrease the heat distortion temperature (HDT), and hence there are inherent limits on the copolymerization ratio and the amounts of the additives that may be employed. Acrylic resins should have a heat distortion temperature of not lower than 70.degree. C. measured in accordance with ASTM D 648, or practical problems are presented in some cases.
As discussed above, molecular weight of the resin, copolymerization ratio, and the kinds and the amounts of additives that may be employed are restricted because of practical requirements for molding workability and heat distortion temperature, etc. Thus, there have been known no molding materials simultaneously satisfying molding workability, practically satisfactory physical properties and solvent resistance. The above-described difficulties are overcome with the present invention.